We are studying the organization and expression of SV40 and adenovirus 2 genomes contained in Ad2-SV40 hybrid virus DNA molecules. The hybrid viruses are also used as models for understanding the behavior of integrated genomes. We are also analyzing the behavior of SV40 and Ad2-transformed or lytically infected cells as a function of the molecular conformation of viral nucleic acids within these cells, and as a function of events which may control (or be controlled by) viral replication, transcription, and translation. Secondly, we are analyzing the state and expression of the endogenous AKR murine leukemia virus genome in mice with varying expression of leukemia and virus production, as well as exploring the evolution, transmission, and relation to leukemogenesis of various murine RNA viruses. The techniques of restriction endonuclease mapping and molecular cloning are being applied to this genetic analysis of ecotropic, xenotropic, amphotropic and recombinant murine leukemia viruses.